Content caching method for satellite terrestrial integrated network with differentiated interest and access mode

1. A content caching method for a satellite terrestrial integrated network with differentiated interests and access modes, comprising: step (1), collecting user proportion information, content popularity distribution information, cache capacity information, and link delay information in the satellite terrestrial integrated network, wherein the satellite terrestrial integrated network comprises base stations, a satellite, base station access users, satellite access users, a ground station, and a cloud server; and a cover area of the satellite terrestrial integrated network is divided into multiple regions, and different regions have different local content popularity distributions; wherein the user proportion information comprises proportion information of the base station access users and the satellite access users in the satellite terrestrial integrated network, and proportion information of users in different regions covered by the satellite terrestrial integrated network; the content popularity distribution information is local content popularity distribution information of different regions covered by the satellite terrestrial integrated network; the cache capacity information comprises base station cache capacity information and satellite cache capacity information; and the link delay information comprises link delay information from the base stations to the base station access users, link delay information from the satellite to the base stations, link delay information from the satellite to the satellite access user, link delay information from the ground station to the satellite, and link delay information from the cloud server to the ground station; step (2), determining a content caching problem of a collaboration of the base stations and the satellite for a delay optimization based on differentiated user interests and differentiated access modes, wherein the delay optimization refers to minimizing an average content retrieval delay of all users in the satellite terrestrial integrated network; and the step (2) comprises: sub-step (2.1), for each region covered by the satellite terrestrial integrated network, calculating, based on a local content popularity distribution in the region, content storage probabilities of a requested content in the base stations, the satellite, the ground station, and the cloud server when the base station access users in the region request any content; and for each region covered by the satellite terrestrial integrated network, calculating, based on the local content popularity distribution in the region, content storage probabilities of the requested content in the satellite, the ground station, and the cloud server when the satellite access users in the region request any content; sub-step (2.2), for each region covered by the satellite terrestrial integrated network, calculating an average content retrieval delay of the base station access users and an average content retrieval delay of the satellite access users, respectively, based on the content storage probabilities and the link delay information; sub-step (2.3), performing weighted summation on the average content retrieval delays of all users in all regions within a coverage area of the satellite terrestrial integrated network to obtain a network average content retrieval delay; and sub-step (2.4), obtaining the content caching problem by taking a content caching strategy of the collaboration of the base stations and the satellite as an optimization variable, minimization of the network average content retrieval delay as an optimization target, and cache capacities of the base stations and the satellite as an optimization limitation condition; step (3), solving the content caching problem to obtain the content caching strategy of the collaboration of the base stations and the satellite; sub-step (3.1), equivalently converting the average content retrieval delay of the satellite access users in different regions into the average content retrieval delay of all the satellite access users under a global content popularity distribution, in such a manner to reduce a calculation complexity of the average content retrieval delay of the satellite access users; sub-step (3.2), reducing a solution space size and reducing a search complexity of the content caching strategy based on solution space analysis; sub-step (3.3), converting the content caching problem into a maximization problem of a delay reduction gain, wherein the delay reduction gain refers to a reduction amount of an average content retrieval delay of the users under a current content caching strategy when no content is cached by the base stations and the satellite; and sub-step (3.4), solving the converted maximization problem of the delay reduction gain to obtain the content caching strategy of the collaboration of the base stations and the satellite; and step (4), caching, based on the obtained content caching strategy, contents in the base stations and the satellite to provide a content service for the base station access users and the satellite access users in the satellite terrestrial integrated network.

2. The content caching method for a satellite terrestrial integrated network with differentiated interests and access modes according to claim 1, wherein the sub-step (3.2) further comprises: sub-step (3.2.1), for each region, calculating fixedly cached contents and selectively cached contents of the base stations in the region based on the local content popularity distribution information, the cache capacity information and the link delay information of the region, wherein the fixedly cached contents refer to contents necessarily cached in the content caching strategy obtained by solving the content caching problem, and the selectively cached contents refer to contents possibly cached in the content caching strategy obtained by solving the content caching problem; and calculating fixedly cached contents and selectively cached contents of the satellite based on the user proportion information, global content popularity distribution information, and the cache capacity information; and sub-step (3.2.2), for each region, reducing a solution space of the content caching strategy of the base stations in the region to a union set of the fixedly cached contents and the selectively cached contents of the base stations in the region, and reducing a search space of the content caching strategy of the base stations in the region to a set of the selectively cached contents of the base stations in the region; and reducing a solution space of the content caching strategy of the satellite to a union set of the fixedly cached contents and the selectively cached contents of the satellite, and reducing a search space of the content caching strategy of the satellite to a set of the selectively cached contents of the satellite.

3. The content caching method for a satellite terrestrial integrated network with differentiated interests and access modes according to claim 2, wherein the sub-step (3.4) comprises the following iterative sub-steps: sub-step (3.4.1), initializing a satellite iterative solution into a set of the fixedly cached contents of the satellite; sub-step (3.4.2), initializing all contents in the set of the selectively cached contents of the satellite to be in an unselected state; sub-step (3.4.3), selecting unselected content from the set of the selectively cached contents of the satellite, and updating a satellite candidate solution corresponding to the unselected content into a union of the satellite iterative solution and selected content; sub-step (3.4.4), calculating, for each region, a content caching strategy when the base stations obtain a maximum delay reduction gain under a satellite candidate solution corresponding to the selected content; sub-step (3.4.5), calculating a corresponding delay reduction gain based on the satellite candidate solution corresponding to the selected content and the calculated content caching strategy of the base stations; sub-step (3.4.6), determining whether there is unselected content in the set of the selectively cached contents of the satellite; when there is unselected content in the set of the selectively cached contents of the satellite, returning to the sub-step (3.4.3); and when there is no unselected content in the set of the selectively cached contents of the satellite, comparing satellite candidate solutions corresponding to all contents in the set of the selectively cached contents of the satellite, and updating the satellite iterative solution to a satellite candidate solution with the maximum delay reduction gain; and sub-step (3.4.7), determining whether the satellite iterative solution reaches an upper limit of the cache capacity of the satellite when there is no unselected content in the set of the selectively cached contents of the satellite; when the satellite iterative solution does not reach the upper limit of the cache capacity of the satellite, returning to the sub-step (3.4.2); when the satellite iterative solution has reached the upper limit of the cache capacity of the satellite, ending the iterative sub-steps; wherein a solved content caching strategy of the satellite is the satellite iterative solution; for each region, a solved content caching strategy of the base stations is the content caching strategy of the base stations calculated in the sub-step (3.4.4) corresponding to the satellite iterative solution.

4. The content caching method for a satellite terrestrial integrated network with differentiated interests and access modes according to claim 3, wherein the sub-step (3.4.4) further comprises: sub-step (3.4.4.1), for each region, initializing the content caching strategy of the base stations in the region to a set of the fixedly cached contents of the base stations in the region; sub-step (3.4.4.2), for each region, calculating, for each content in a set of the selectively cached contents of the base stations in the region, a delay reduction amount for the base station access users in the region to retrieve the content when the content caching strategy of the satellite is a given satellite candidate solution in a case where the base stations cache the contents as compared with a case where the base stations do not; and sub-step (3.4.4.3), for each region, sorting the calculated delay reduction amounts for all contents in the set of the selectively cached contents of the base stations in the region, and adding the contents with a maximum delay reduction amount into the base stations in the region to be cached until a number of the cached contents of the base stations in the region reaches an upper limit of the cache capacity of the base stations.